1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission for a vehicle. More specifically, it relates to a control for starting and stopping an automatic transmission that allows hill-hold control to be carried out along with neutral control.
2. Description of Background Art
According to conventionally proposed controls of an automatic transmission of a vehicle as disclosed in Japanese Patent Application Laid-open No. Sho 63-106449, an input clutch is disengaged to enhance fuel efficiency and a hill-hold brake is engaged at the same time to prevent the vehicle from backward movement on a steep slope when the vehicle is halted in the first drive range thereof. The control method for disengaging the input clutch will be referred to hereafter as "neutral control" and the control method for engaging the hill-hold brake will be referred to hereafter as "hill-hold control". The input clutch in the automatic transmission is disengaged and engaged gradually to reduce disengagement and engagement shocks.
However, the hill-hold brake in the prior art control apparatus of the vehicle as described above is disengaged and engaged abruptly in comparison with the input clutch, which causes a disengagement shock and an abrupt change in resisting force serving to prevent the vehicle from rolling backward.
When the driver releases the foot brake and then depresses the accelerator pedal to start the vehicle in a neutral-control state on a steep slope, for example, the input clutch is engaged gradually but the hill-hold brake is disengaged abruptly. At that time, if the hill-hold brake is disengaged abruptly after substantial engagement of the input clutch, a 2-to-1 shock (that is, a disengagement shock) is produced. This is caused by the hill-hold brake also serving as a transmission brake for achieving a state of a second ratio and the hill-hold brake temporarily creating the state of the second ratio and then switching to a state of a first ratio. If the hill-hold brake is disengaged abruptly before substantial engagement of the input clutch, on the other hand, the braking force preventing backward movement of the vehicle is abruptly removed before engine torque is transmitted through the clutch to urge the vehicle forward. The absence of both a creep force generated by the engagement of the input clutch and a hill-hold force generated by the hill-hold brake allows the vehicle to roll backward under the force of gravity.
It should be noted that the neutral and hill-hold controls are carried out only when the vehicle is halted or the speed thereof is close to zero, not when the vehicle is running at a normal speed.
The disengagement shock or the backward movement due to an abrupt change in resisting force serving to prevent the vehicle from rolling backward produced during the prior art neutral and hill-hold controls are more noticeable to the driver than any shock or change in force caused during the normal running state of the vehicle. This brings a sense of uneasiness to the driver and could cause accidental collision with a car to the rear.